Re: [PATCH v6 6/6] swiotlb: Reinstate page-alignment for mappings >= PAGE_SIZE

[Petr Tesařík]

On Fri, 8 Mar 2024 16:08:01 +0000
Robin Murphy <robin.murphy@xxxxxxx> wrote:

> On 2024-03-08 3:28 pm, Will Deacon wrote:
> > For swiotlb allocations >= PAGE_SIZE, the slab search historically
> > adjusted the stride to avoid checking unaligned slots. This had the
> > side-effect of aligning large mapping requests to PAGE_SIZE, but that
> > was broken by 0eee5ae10256 ("swiotlb: fix slot alignment checks").
> > 
> > Since this alignment could be relied upon drivers, reinstate PAGE_SIZE
> > alignment for swiotlb mappings >= PAGE_SIZE.  
> 
> This seems clear enough to keep me happy now, thanks! And apologies that 
> I managed to confuse even myself in the previous thread...
> 
> Reviewed-by: Robin Murphy <robin.murphy@xxxxxxx>

I thought we agreed that this stricter alignment is unnecessary:

  https://lore.kernel.org/linux-iommu/20240305140833.GC3659@xxxxxx/

But if everybody else wants to have it...

Petr T

> > Reported-by: Michael Kelley <mhklinux@xxxxxxxxxxx>
> > Signed-off-by: Will Deacon <will@xxxxxxxxxx>
> > ---
> >   kernel/dma/swiotlb.c | 18 +++++++++++-------
> >   1 file changed, 11 insertions(+), 7 deletions(-)
> > 
> > diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
> > index c381a7ed718f..c5851034523f 100644
> > --- a/kernel/dma/swiotlb.c
> > +++ b/kernel/dma/swiotlb.c
> > @@ -992,6 +992,17 @@ static int swiotlb_search_pool_area(struct device *dev, struct io_tlb_pool *pool
> >   	BUG_ON(!nslots);
> >   	BUG_ON(area_index >= pool->nareas);
> >   
> > +	/*
> > +	 * Historically, swiotlb allocations >= PAGE_SIZE were guaranteed to be
> > +	 * page-aligned in the absence of any other alignment requirements.
> > +	 * 'alloc_align_mask' was later introduced to specify the alignment
> > +	 * explicitly, however this is passed as zero for streaming mappings
> > +	 * and so we preserve the old behaviour there in case any drivers are
> > +	 * relying on it.
> > +	 */
> > +	if (!alloc_align_mask && !iotlb_align_mask && alloc_size >= PAGE_SIZE)
> > +		alloc_align_mask = PAGE_SIZE - 1;
> > +
> >   	/*
> >   	 * Ensure that the allocation is at least slot-aligned and update
> >   	 * 'iotlb_align_mask' to ignore bits that will be preserved when
> > @@ -1006,13 +1017,6 @@ static int swiotlb_search_pool_area(struct device *dev, struct io_tlb_pool *pool
> >   	 */
> >   	stride = get_max_slots(max(alloc_align_mask, iotlb_align_mask));
> >   
> > -	/*
> > -	 * For allocations of PAGE_SIZE or larger only look for page aligned
> > -	 * allocations.
> > -	 */
> > -	if (alloc_size >= PAGE_SIZE)
> > -		stride = umax(stride, PAGE_SHIFT - IO_TLB_SHIFT + 1);
> > -
> >   	spin_lock_irqsave(&area->lock, flags);
> >   	if (unlikely(nslots > pool->area_nslabs - area->used))
> >   		goto not_found;  
>